1. Field of the Invention
The present invention relates to the field of fabrication of semiconductor devices. More specifically, the invention relates to the fabrication of silicon-germanium semiconductor devices.
2. Related Art
In a heterojunction bipolar transistor, or HBT, a thin silicon-germanium layer is grown as the base of a bipolar transistor on a silicon wafer. The silicon-germanium HBT has significant advantages in speed, frequency response, and gain when compared to a conventional silicon bipolar transistor. Speed and frequency response can be compared by the cutoff frequency which, simply stated, is the frequency where the gain of a transistor is drastically reduced. Cutoff frequencies in excess of 100 GHz have been achieved for the HBT, which are comparable to the more expensive GaAs. Previously, silicon-only devices have not been competitive for use where very high speed and frequency response are required.
The higher gain, speeds, and frequency response of the HBT have been achieved as a result of certain advantages of silicon-germanium not available with pure silicon, for example, narrower band gap, and reduced resistivity. Silicon-germanium may be epitaxially grown on silicon wafers using conventional silicon processing and tools, and allows one to engineer device properties such as the band gap, energy band structure, and mobilities. For example, it is known in the art that grading the concentration of germanium in the silicon-germanium base builds into the HBT device an electric field, which accelerates the carriers across the base, thereby increasing the speed of the HBT device compared to a silicon-only device. One method for fabricating silicon and silicon-germanium devices is by chemical vapor deposition (“CVD”). A reduced pressure chemical vapor deposition technique, or RPCVD, used to fabricate the HBT device allows for a controlled grading of germanium concentration across the base layer. As already noted, speeds in the range of approximately 100 GHz have been demonstrated for silicon-germanium devices, such as the HBT.
A polycrystalline silicon emitter can be formed above the epitaxially grown single crystal silicon-germanium base. There are several possible methods of forming a polycrystalline silicon emitter. For example, a simple way would be to deposit polycrystalline silicon above the epitaxially grown single crystal silicon-germanium base, mask the emitter area with photoresist, and etch to form the emitter. A problem with this approach is that it is difficult to stop the etch of the emitter without etching into the single crystal silicon-germanium base. Etching into the single crystal base can cause, for example, damage in the form of pitting and recessing the silicon-germanium base. Pitting can lead, for example, to loss of base doping control and loss of device integrity. It is necessary to stop the etch precisely at the interface between the polycrystalline silicon emitter and the single crystal silicon-germanium base, but both emitter and base are substantially the same material at the interface. Thus, it is difficult, if not impossible, to selectively etch the polycrystalline silicon emitter without etching the single crystal silicon-germanium base. A different approach to the problem is needed.
One such approach is to form a layer of some material which can be selectively etched to the single crystal silicon-germanium base and open a “window” in that material in which to deposit the polycrystalline silicon for the emitter. After the polycrystalline silicon is deposited for the emitter, the excess material is etched away selectively to the silicon-germanium base, forming the polycrystalline silicon emitter above the single crystal silicon-germanium base.
It is critical to the proper performance of the HBT that the dimension of the width and length of the emitter be very accurately controlled. A dimension which critically affects the performance of devices on a semiconductor wafer, for example the emitter width of the HBT, is generally referred to as critical dimension, or “CD.” Chip manufacturers calculate a CD “budget” for the semiconductor wafer, which is the allowable variation for critical dimensions on the wafer surface. As feature sizes for devices on the surface of the semiconductor wafer become smaller, it becomes more difficult to accurately control the dimensions of features such as the emitter window discussed above.
Control of dimensions is difficult because each step in the photolithographic patterning process contributes variations. For example, unwanted variation in dimension of a feature may be caused by defects in the photomask; reflectivity of a surface of the material below the photoresist, referred to as “subsurface reflectivity,” which causes scattering of the light used to expose the photoresist; adhesion problems between an antireflective coating and the wafer and photomask; or poor matching of index of refraction between an antireflective coating and the photomask. Thus, as feature sizes become smaller, the CD budget becomes stricter, necessitating more accurate control over critical dimensions, for example the emitter width of the silicon-germanium HBT. In the case of the silicon-germanium NPN HBT control of the emitter width is essential to the performance of the device.
In the above approach of opening an emitter window in a layer of material which can be selectively etched to the single crystal silicon-germanium base, one possible material is silicon nitride. Critical dimension control of the emitter window is difficult with silicon nitride because silicon nitride is subject to subsurface reflectivity, as discussed above. Control of the critical dimension of the emitter is also difficult with silicon nitride because the wet clean process used to prepare the surface of silicon-germanium base prior to deposition of the polycrystalline silicon emitter in the emitter window laterally etches the silicon nitride, increasing the width of the emitter window and thereby causing unwanted variation in the critical dimension. Thus, the use of silicon nitride does not provide a satisfactory solution to the problem of forming a polycrystalline silicon emitter with critical dimension control.
Another possible material to use in the above approach of opening an emitter window in a layer of material which can be selectively etched to the single crystal silicon-germanium base is silicon oxynitride. Silicon oxynitride has less subsurface reflectivity than silicon nitride, but it is difficult to selectively etch silicon oxynitride to silicon oxide, leading to pitting and recessing damage of the silicon-germanium base as discussed above. In addition, unwanted silicon oxynitride is difficult to remove after the emitter has been formed. Unremoved silicon oxynitride can degrade performance of the silicon-germanium HBT. Thus, the use of silicon oxynitride does not provide a satisfactory solution to the problem of forming a polycrystalline silicon emitter with critical dimension control.
There is thus a need in the art for a polycrystalline silicon emitter structure in which the width dimension is precisely controlled. There is also need in the art for a smaller polycrystalline silicon emitter structure. Further, there is a need in the art for a polycrystalline silicon emitter structures as small as the resolution of the photolithography process will allow. Moreover, there is need in the art for a method of fabricating a polycrystalline emitter structure in which precise control of critical dimension enables the fabrication of a polycrystalline emitter structure as small as the resolution of the photolithography process will allow.